cAMP Signaling in N. crassa
Explore and order pathway-specific siRNAs, real-time PCR assays, and expression vectors. View pathway information and literature references for your pathway.
  • Click on your proteins of interest in the pathway image or review below
  • Select your genes of interest and click "add selection"
  • When you have finished your gene selection, click "Find Products" to find assays, arrays, or create custom products
Download Image Terms of Use Download PPT
Pathway Navigator
cAMP Signaling in N. crassa

The ascomycete N. crassa (Neurospora crassa) has defined asexual and sexual cycles. N. crassa is heterothallic and has two mating types, A and a. Under nutrient-rich conditions like Carbon, etc, N. crassa proliferates by the extension and branching of multinucleate vegetative hyphal cells to form a multicellular mycelium (Ref.1). In response to nutrient deprivation, desiccation and light, N. crassa initiates asexual conidiation. The major determinants regulating the onset of conidiation are the availability of Carbon source and light. The sexual cycle initiates in response to Nitrogen starvation. Heterotrimeric G-proteins play a critical role in regulating growth and differentiation in the filamentous fungi N. crassa. Filamentation, conidiation, morphogenesis, mating and stress tolerance in N. crassa are controlled by cAMP (Cyclic Adenosine 3,5-monophosphate) signaling consisting of G-protein subunits (GNA-1 (Guanine Nucleotide-Binding Protein-Alpha1 Subunit), GNA-2, GNA-3, GNB-1 (Probable Guanine Nucleotide-Binding Protein-Beta Subunit) and GBG (Guanine Nucleotide-Binding Protein-Gamma Subunit)); AC (Adenylate Cyclase)/Cr-1; and PKA (Protein Kinase-A). N. crassa PKA regulates hyphal growth polarity, possibly through organization of the cytoskeleton and Glucose-regulated gene expression. MAPK (Mitogen-Activated Protein Kinase) and PKA-related Kinase-mediated pathways also regulate conidiation. However, how the signaling is coordinated between these pathways remain to be elucidated. Apart from nutrient sensing, the N. crassa, G-proteins are also involved in Pheromone (CCG-4 (Clock-Controlled Pheromone-CCG-4) and MFa-1 (Pheromone-MFa-1)) sensing (Ref.1 & 2).

GPCRs (G-Protein-Coupled Receptors) are a family of seven transmembrane helix receptors that bind to ligands such as pheromones. GPCRs are associated with heterotrimeric G-proteins, consisting of Aplha (GNA-1, GNA-2 and GNA-3), Beta (GNB-1) and Gamma (GBG)-subunits. Pheromone Receptors (Pre1 and Pre2) act synergistically with G-proteins like GPCRs. G-proteins regulate both cAMP dependent and MAPK signaling in N. crassa. cAMP-dependent functions are regulated through activation of PKAC, the Catalytic subunit of PKA, while cAMP-independent processes are modulated by MAPK pathways. In the inactive state, the heterotrimeric G-proteins are docked at the receptor and GDP is bound to the G-Alpha subunit. Binding of a ligand activates the receptor, leading to the exchange of GDP for GTP on G-Alpha and the subsequent dissociation of G-Alpha-GTP from the G-Beta/G-Gamma moiety (Ref.2). The response is terminated and the cycle is completed with hydrolysis of GTP by G-Alpha. The GDP-bound G-Alpha protein re-associates with G-Beta/G-Gamma and the heterotrimer is then able to bind to the receptor to wait the next cycle of activation. During the sexual cycle, GNA-1 is necessary for female fertility, while GNA-3 mediates sexual spore (Ascospore) maturation. GNA-3 and GNA-2 modulates perithecial orientation and Ascospore viability via a cAMP-independent mechanism. GNA-1 acts as a direct regulator of AC/Cr-1 activity, while GNA-3 affects this enzyme indirectly through modulation of AC/Cr-1 protein levels. cAMP influences apical extension, conidiation and stress resistance. GNA-2 plays a minor role in regulation of AC/Cr-1 activity (Ref.2 & 3).

Regulation involves direct interaction between the G-Alpha and AC/Cr-1 and/or indirect effects resulting from the action of freed G-Beta/G-Gamma dimers on the enzyme. Stimulation by G-Alpha increases cAMP levels, leading to activation of PKA, while inhibition by G-Alpha decreases intracellular cAMP levels, leading to down-regulation of PKA. Binding of cAMP to the Regulatory Subunit, MCB releases the Catalytic subunit PKAC, to phosphorylate the target proteins involved in cAMP-regulated processes. Hence inactivation of the Regulatory subunit activates the catalytic PKA subunit in N. crassa. PKA Regulatory subunit, MCB, is involved in mating, conidiation and stress tolerance (Ref.2). GNA-1 and in a minor capacity, GNA-2 regulate female fertility, possibly through modulation of a pheromone response MAPK cascade including the MAPKKKs (MAPKK Kinases) like Nrc-1 and Os-4/Putative SSK22 Like MAPKK Kinase; MAPKK (MAPK Kinase) like Os-5/Putative PBS2 Like MAPK Kinase; and MAPK (like MAP Kinase-Sty1/MAP Kinase-Spc1 and MAK-2). GNA-1 and Nrc-1 also regulate conidiation. The functions of the G-Beta/G-Gamma are difficult to define; however, G-Beta/G-Gamma may regulate MAPK signaling pathways, such as that containing Nrc-1 (Ref.3).

In summary, a cAMP signaling regulates morphology, conidiation, mating and responses to heat shock and oxidative stress in N. crassa. Although cAMP facilitates growth and development in the filamentous fungi, only one heterotrimeric G-Alpha protein has been completely known to modulate cAMP levels. Further analysis will reveal whether regulation of AC by multiple G-Alpha subunits is a general paradigm in filamentous fungi.  Regulation of morphology and conidiation involve integrity of cellular growth polarity and cytoskeletal organization (Ref.1). Two parallel signaling pathways (MAPK and cAMP), as is the case in S. cerevisiae (Saccharomyces cerevisiae), S. pombe (Schizosaccharomyces pombe), C. albicans (Candida albicans), C. parasitica (Cryphonectria parasitica), M. grisea (Magnaporthe grisea), U. maydis (Ustilago maydis) and C. neoformans (Cryptococcus neoformans), also exist in N. crassa. Further studies are required to establish the relationships among these signaling pathways. N. crassa is genetically tractable, thus it is an excellent model organism for the study of signaling cascades regulating morphogenesis and differentiation in filamentous fungi (Ref.2).